Nowadays, a motor is an indispensable power component. The motor can convert electric energy into kinetic energy, so it is widely used in the apparatus such as a machine tool, water pump, light industry machinery, wind power generation, hydropower generation, electric vehicle, etc. Due to high demand in the market, the design and manufacture of various motors and the technology of driving and controlling these motors are rapidly developed.
In general, the driving methods of the motor can be divided into asynchronous drive and synchronous drive. The asynchronous drive is performed by the interaction between the magnetic flux and the induced current generated by the slip between the rotor and the stator so as to control the rotation of the rotor. The synchronous drive is performed by attracting the negative pole of the rotor magnetic field by the magnetic field of the stator, so that the rotor, following the magnetic field of the stator, rotates at the same speed as that of the magnetic field of the stator. The advantages of the asynchronous driving method comprise less being impacted by the environment and having a high starting torque; however, its power factor is low. The synchronous driving method has the advantage of high power factor; however, it is easily impacted by the environment so that it easily fails in the start-up phase or has poor performance at low speeds.